This invention relates to a resistor material, resistors made from the resistor material, and a method for making the resistors, and in particular is directed to improvements in tin oxide resistor materials.
Electrical resistor materials comprising a mixture of a glass frit and finely divided particles of tin oxide (SnO.sub.2) have achieved significant commercial acceptance. This type of resistor material, a resistor comprising a substrate with fired resistor material on the substrate, and a method for making the material and the resistor are described in U.S. Pat. No. 4,322,477. Resistors made from resistor material comprising glass frit and tin oxide have many desirable features including:
1. A wide range of resistivities from about 700 ohms per square to about 10 megohms per square; PA0 2. High stability with changes in temperature, i.e. a low temperature coefficient of resistance, often within the range of .+-.200 ppm/.degree.C.; PA0 3. Compatability with commonly used, low cost terminations, such as copper terminations; PA0 4. Low cost, i.e. tin oxide is less expensive than noble metals such as gold, silver, ruthenium, and platinum used in other resistor materials; and PA0 5. Stability under use conditions, i.e. the resistor is generally unaffected by moisture, humidity, static charges, and temperature cycling.
Resistor materials comprising tin oxide and glass frit providing a resistance less than 700 ohms per square are unavailable unless the resistor material is applied in a thickness greater than about 1 mil. However, if the thickness of the resistor material is greater than about 1 mil, problems occur, including (1) difficulty in laser trimming and firing the resistor, (2) low stability with changes in termperature, i.e. a high temperature coefficient of resistance; and (3) thermal mismatch problems between the resistor material and the underlying substrate.
Accordingly there is a need for a resistor material having the advantages of a glass frit/tin oxide resistor material, and that when applied in a thickness of less than about 1 mil is also useful for producing resistors having a resistivity of less than 600 ohms per square.